The present invention relates generally to semiconductor devices, and more particularly, to a method and apparatus for light to frequency conversion.
A number of optoelectronic systems applications require an accurate measurement of the intensity of a light beam or incident light, the measurement being performed over large ranges of input signal amplitude and wavelength. It is desired that the measurement accuracy be maintained over a wide range of environmental conditions. In addition, it is desired that the measurement be made in a minimal volume as dictated by packaging considerations and at a cost commensurate with consumer type systems.
Typical system requirements dictate that an incident light intensity needs to be converted to a digital form for use by a digital processor. To accomplish this, the light intensity is converted to an electrical form that can be digitized. For example, one technique applicable to sampled data processor systems includes converting the light intensity to a voltage that can be applied to an A/D Converter (ADC). This method however requires an additional analog block (i.e., the ADC), but can be used in those applications requiring a higher bandwidth. A second technique, applicable to very low bandwidth applications, includes directly converting the light intensity into a frequency that can be counted by a digital processor.
U.S. Pat. No. 5,850,195 entitled xe2x80x9cMONOLITHIC LIGHT-TO-DIGITAL SIGNAL CONVERTER,xe2x80x9d issued Dec. 15, 1998, discloses a converter having a switched capacitor oscillator in which the reference function is included in the oscillator circuit. The switched capacitor oscillator requires that calibration be accomplished by trimming the capacitors of the oscillator. However, the oscillator has a high level of parasitic capacitance which limits its high frequency performance. In addition, certain applications, including infrared incident intensity, for example, require a different temperature coefficient which cannot be implemented in an efficient manner in the oscillator circuit of the ""195 converter.
It would be desirable to provide a light to frequency converter and light to frequency conversion technique for overcoming the above discussed problems in the art.
According to one embodiment of the present disclosure, a light to frequency converter includes a temperature coefficient generator, a programmable gain amplifier, and a current controlled oscillator having at least one photodiode configured to receive incident light. The at least one photodiode is configured for generating a photodiode control current. The temperature coefficient generator outputs a bandgap reference voltage with temperature coefficient compensation (VBG_TC) in response to a bandgap reference voltage (VBG). The programmable gain amplifier is responsive to the bandgap reference voltage with temperature coefficient compensation (VBG_TC) for outputting an oscillator reference voltage (VREF). Lastly, the current controlled oscillator further includes a switching capacitor configured to provide a feedback current. The current controlled oscillator is responsive to the oscillator reference voltage (VREF), the photodiode control current, and the feedback current for producing an output signal having a frequency proportional to an intensity of the incident light. In addition, the bandgap reference voltage with temperature coefficient compensation modifies a temperature coefficient of the switched capacitor feedback current to match a temperature coefficient of the photodiode control current. A light-to-frequency conversion methodology and a light-to-frequency controller apparatus are also disclosed.
According to another embodiment, the light to frequency converter operates with the use of a bandgap voltage generator, the bandgap generator not including temperature coefficient compensation.